Goal: To understand the
basics of interference
Objectives:
1) What is interference
2) Constructive interference
3) Destructive interference
4) Path differences for complete
interference
What is interference?
• Interference is when a defender attacks a
receiver before the ball reaches him.
• Err wait sorry wrong sport.
• Interference is when 2 waves reach the same
spot.
• These waves will add together – thus interfering
with one another.
• Depending on how they interfere this can be a
good thing or a BAD thing.
Constructive interference
• If two waves land on top of each other
they can add together if their positives and
negatives match up (this is called “in
phase”).
• If the waves peak at exactly the same
time, they will add together making a
bigger wave.
• This is called constructive interference.
Destructive interference
• If two waves land on top of each other they can
subtract if their positives meet with the others
negatives (this is called “out of phase”).
• If the waves peak at exactly opposite times, they
will subtract making a smaller wave.
• If the waves have the same amplitude (or
strength) then they can COMPLETELY cancel
each other out!
• This is called destructive interference.
Uses?
• Think of 1 possible GOOD use for both
constructive and destructive interferrence.
Path differences for complete
interference
• If a source emits photons of a specific
wavelength in 2 different directions it is
possible (due to reflection or something
else) that those two paths will cross.
• When they cross you can possibly get
interference.
• However, what type of interference will you
get?
Depends
• The interference type depends on the difference
in distances that each wave has to take.
• Lets say a radio wave is 1 m long.
• What must be true about the difference in
distances if two paths meeting produce
constructive interference?
• What about for destructive interference?
Depends
• The interference type depends on the difference in
distances that each wave has to take.
• Lets say a radio wave is 1 m long.
• What must be true about the difference in distances if
two paths meeting produce constructive interference?
• The difference in paths has to be equal to an exact
number (i.e. an integer) of wavelengths.
• So, 1 m, 2 m, 17m, ect, but not 1.1 m.
• What about for destructive interference?
• Has to be a difference of half waves – such as 0.5 m,
1.5m, 2.5 m, 175.5 m, but not 0.4 m, or 0.6 m, and def
not 3.0 m.
Example
• This is from HW 10
• A radio tower emits radio waves with a
wavelength of 400 m.
• These waves travel 15.3 km to a radio receiver.
• One path is directly to the receiver.
• The 2nd path goes past the receiver and reflects
of the hills behind the receiver and come back.
• If no the reflection causes no phase change in
the waves then what is the smallest distance
away the hills can be from the receiver if total
destructive interference is caused?
Example
• A radio tower emits radio waves with a wavelength of
400 m.
• These waves travel 15.3 km to a radio receiver.
• One path is directly to the receiver.
• The 2nd path goes past the receiver and reflects of the
hills behind the receiver and come back.
• If no the reflection causes no phase change in the waves
then what is the smallest distance away the hills can be
from the receiver if total destructive interference is
caused?
• The path difference has to be a half wave, or 200 m.
• But, since the radio waves have to go to the hill and back
the path difference is twice the distance to the hills.
• So, the distance to the hills is 100 m.
Conclusion
• We quickly looked at what constructive
and destructive interferences are.
• We saw to get them we needed to add
waves either in phase or out of phase.
• If they all come from the same place then
we need to have the path differences be
some number of wavelengths for
constructive interference, or some half
number for destructive.